1. Field of the Invention
The present invention relates to pivoted, straight edge measuring means and more particularly to an instrument for determining length along a side of bent material, as measured from the virtual apex of the angle of bend regardless of the radius of the bend, and optionally for measuring the degree of that angle of bend.
2. Background Information
There has been a long time need for an instrument for measuring the length of a leg of bent material such as sheet metal, when the measurement must be taken from the virtual apex of the angle to which the material is bent. The apex, for all practical purposes is at an imaginary location that is beyond the ending of the joined legs. Putting it another way, the apex of the radius of the bend can be expected to fall short of the apex of the angle, unless the radius is "0" and the bend therefore perfectly angular.
Knowledge of the leg's length at its surface is required for sheet metal manufacture of modular subassemblies for enclosures having a plurality of flat sides. In order to approach perfect fit, one would want to be able to have as one reference, this geometrical constant which is unaffected by the slight variations that occur in bend radius from sheet to sheet.
It is often additionally advantageous to know the bend angle of the material so that it can be compared with the angles formed between other sides of the enclosure, in order to provide a symetrically formed enclosure.
One method presently used to make the measurement involves laying the bent material on a large flat surface so that the outer face of one leg of the bend is in intimate contact with the flat and then bringing a straight edge ruler to the other leg's face, holding it perpendicular to the angle of bend axis of rotation, sliding it down along the inclined face until it just meets the flat surface thus forming the angle of bend between the straight edge ruler and the flat plate.
This is not as easy a task as it may seem for a single operator. If the bent sheet metal module is light weight, the weight of the ruler against the inclined face will tend to shove the material out of position. The operator must simultaneously position the ruler, hold the sheet metal which may also deform under the weight of the ruler and take a measurement reading.
If the item to be measured is large and cannot be placed on a table to be measured, the operator has to hold two rulers in a plane which is perpendicular to the bend angle's axis of rotation, against the two outer faces of the bent material, and manipulate them until they just meet at the virtual point of intersection of the two faces without distorting the faces, and then take the measurement reading.
There are instruments presently available which can be pressed into service to take the type of measurement herein discussed but they have inherent limitations which prevent their use for all but the larger bend radiuses which may be encountered.
For example; The COMBINED PROTRACTOR AND RULE of W. S. Adams, U.S. Pat. No. 999,725, patented Aug. 8, 1911 includes a pair of hinged legs, graduated along their length for establishing measurements and having locator edges which physically intersect the pivotal connection. This instrument may be used to make the measurements under discussion above, by placing the edges which intersect the pivotal connection, against and parallel to the two angled faces and along a plane perpendicular to the bend angle's axis of rotation. The scale values other than "0" commence at the axis of rotation but values may be interpolated from them.
The instrument also includes a protractor scale attached to one of the hinged legs, centered on the axis of rotation, and a reference mark on the other leg for registering the angle to which the edges are positioned.
The Adams instrument and those of its type cannot however provide all the measurement service for which the present invention is intended. Specifically, it cannot measure materials having the shorter bend radiuses because the smaller the bend radius the closer the apex of the radius approaches to coinciding with the apex of the bend angle. An interference will therefore occur between portion of the hinge and/or protractor rule means and the apex of the material's bend radius.
U.S. Pat. No. 3,521,365 awarded July 21, 1970 to J. E. O'Neal for a DUCT MEASURING INSTRUMENT, reduces chance of interference between the hinge and bend radius by displacing the axis of rotation of the instrument's legs from their locator surfaces by the thickness of the duct material for which measurements are taken. By providing a measured value which is adjusted for the thickness of the duct material, the O'Neal invention defeats the purpose of the present invention.